Demands for higher quality images in complementary metal-oxide-semiconductor (CMOS) image sensors have been increasingly raised in recent years, and miniaturization of pixels is indispensable to realize high-resolution images. In addition, a low-back-type CMOS image sensor has also been sought, and the problem of peripheral light loss which is referred to as shading in the CMOS image sensor has become more serious. To solve this problem, a structure such as a digital microlens has been proposed recently for use as a microlens to be arranged at each pixel.
A digital microlens provides a refractive-index-distributed-type lens that controls an overall refractive index distribution by adjusting the cycle and pitch of a grating-type high-refractive-index material and a low-refractive-index material (which may be air). Accordingly, it has been considered that adverse effect caused by a shading effect can be improved more than before by making a distribution of the refractive index different between the center and the periphery of a pixel.
The digital microlens in the past, however, uses existing materials and has a problem that a light collecting characteristic would be deteriorated, because a sufficient refractive index cannot be attained in the course of miniaturization of pixels.
A metamaterial has been proposed as a high-refractive-index material. This high-refractive-index material has been provided by layering a metal and an insulating material even in the terahertz band, and by arranging an I-type pattern periodically to thereby realize a material having a high refractive index widely on a very “positive” side.